1. Field of the Invention
The present invention relates to an optical recording media capable of achieving high-density recording.
2. Description of the Related Art
As optical recording media capable of recording information at a high density using light, a phase-change optical recording media capable of rewriting many times and a write-once optical recording media capable of recording only once have been mainly used.
The phase-change optical recording media comprises a recording layer formed of a material in which phase a change by irradiation with light brings about change in reflectance. The phase-change recording layer containing, for example, Ge, Sb, Te, In or Ag as a main component is melted upon irradiation with high-power, short-pulsed light and made amorphous in a cooling stage in which the amorphous region becomes a recording mark. If the recording mark is irradiated with low-power, long-pulsed light, the recording mark is heated to a temperature higher than the crystallizing temperature and, then, is cooled slowly. As a result, the amorphous mark is crystallized, which corresponds to erasure. The recording media using the phase-change recording layer makes it possible to rewrite data by repeating the operations described above. The data can be read by detecting difference in reflectance between the amorphous recording mark and the crystalline space. It follows that the magnitude of the reflectance difference is determined by change in an optical constant of the recording material accompanying the phase change. The recording material used nowadays, which has been found as a result of research over a long time, exhibits very large changes in optical characteristics in accordance with the phase change. However, where recording density is further enhanced in future and the recording mark is made smaller, detection of the reflectance change for the recording material noted above is expected to be hard.
On the other hand, known write-once optical recording media have a recording layer formed of an inorganic material including chalcogenide elements such as a Te compound, or a recording layer formed of a recording material prepared by dispersing in an organic material a dye such as a cyanine derivative, a phthalocyanine derivative, a porphyrin derivative or a metal porphyrin derivative. The inorganic recording layer is formed by a deposition method represented by a dry process such as vacuum evaporation or sputtering. The organic recording layer is formed by a wet process such as spin coating or electrolysis. In the spin coating, a solution prepared by dissolving an organic dye in a solvent such as dichloroethane is dripped onto a substrate while rotating the substrate so as to form a thin film on the substrate. The spin coating is widely accepted as an inexpensive method of forming an organic recording layer. All write-once discs available on the market nowadays operated with a red laser diode, such as CD-R and DVD-R, use the aforementioned organic dyes for the recording layer.
Almost all write-once discs having a recording layer containing an organic dye utilize a recording mechanism with local rupture of the recording layer. To be more specific, when light is focused in a size of about 1 μm on a recording layer by an objective lens, the light is absorbed by the dye and converted into heat, which locally evaporates the dye or deforms a material in contact with the dye. As a result, when light is focused on that portion of the recording layer in read time, the light is scattered to lower reflectance. It follows that the portion is recognized as a recording mark. However, if the dye has excessively high absorbance for the wavelength of the light source, the dye is decomposed by irradiation with read light. Thus, the recording layer should have significantly high absorbance for the wavelength of the light source in order to convert the absorbed light into heat efficiently, while the recording layer should not have excessively high absorption in order to avoid failure of the data in read time. Such being the situation, the write-once discs such as CD-R and DVD±R uses a cyanine-based or phthalocyanine-based dye having an absorption peak in the vicinity of 780 nm or 650 nm, i.e., the wavelength of the light source.
The optical recording media including a rewriting type and write-once type are always required to enhance recording density. Under the circumstances, enhancement in recording density has been pursued by means of: (1) shortening in the wavelength of the light source, and (2) increase in the numerical aperture (NA) of the objective lens. Each of these means is intended to reduce a beam spot focused on the optical recording media so as to form smaller recording marks and read out the recording marks. This is because the recording capacity can be increased by reducing the recording mark. However, since reduction of the mark size is limited, it is necessary to take another measure.
One of approaches to achieve a higher recording density is to increase the difference in optical characteristics between the recording portion and the non-recording portion. The phase-change material used nowadays in the rewritable DVD contains Ge, Sb, Te, In or Ag as a main component, and the composition thereof is chosen to permit a very high reflectance difference between the recording portion and the non-recording portion. Also, the organic dye material used in the write-once DVD, which has also been found as a result of extensive research over a long time, is optimized to achieve the highest reflectance change for the wavelength of the light source. Further, although next-generation recording media of rewritable and write-once types operated under a wavelength of 405 nm are being developed, there has not yet been found a material exhibiting optical change exceeding the level of the recording material used nowadays.